Two standard types of PBSs include polarizing beams splitters containing birefringent materials (often referred to herein as birefringent PBSs) and polarizing beam splitters using thin-film coatings (often referred to herein as coated PBSs). Generally, coated PBSs are much less expensive and more durable than birefringent PBSs. Additionally, all the materials in a coated PBS can be “factory-made” to a high degree of purity, but calcite, which is the most commonly used birefringent material in birefringent PBSs, currently cannot be artificially manufactured and supplies of calcite found in nature are dwindling.
FIG. 1 shows a cross-section of a coated PBS 100 including a thin-film coating 120 between two pieces of isotropic glass 110 and 130. Glass pieces 110 and 130 are prisms having cross-sections that are isosceles right triangles. Thin-film coating 120 is on one piece of glass (e.g., prism 110) and a glue layer that binds the pieces is on the other piece of glass (e.g., prism 130).
Thin-film coating 120 generally includes multiple layers of two or more materials having different indices of refraction. The layer materials, the number of layers, and the thickness of each layer are selected to transmit light having a first linear polarization P and reflect light having a second linear polarization S.
As illustrated in FIG. 1, an input beam IN containing components with both polarizations P and S is incident normal to the surface of glass 110 and at 45° to the normal of thin-film coating 120. If thin-film coating 120 is properly constructed, a beam TOut, which is transmitted through thin-film coating 120, predominantly contains light having the first polarization P, and a beam ROut, which is reflected from thin-film coating 120, predominantly contains light having the second polarization S. Generally, each output beam TOut and ROut will have polarization components with both polarizations P and S. The ratio of the intensity of the predominant polarization component P or S to the other polarization component S or P is commonly referred to as the extinction ratio.
Coated PBSs generally do not perform as well as birefringent PBSs. Many birefringent PBSs can provide extinction ratios greater than 10,000:1 for both transmitted and reflected beams. Most commercially available coated PBSs produce one “good” polarization (usually the transmitted beam) having an extinction ratio of about 500:1 or better and a not-as-good polarization having an extinction ratio of less than about 200:1. Most optics catalogs and manufacturers specify the extinction ratio of the transmitted beam, and the extinction ratio for the reflected beam (if indicated) is relatively poor.
Many applications, including interferometers and some fiberoptic gyroscopes, require two beams with polarizations that are highly linear and orthogonal, for example, output beams from a PBS having extinction ratios greater than about 1,000:1. These applications have commonly used birefringent beam splitters or added “clean-up” polarizers to the outputs of a coated PBSs. Adding polarizers to the output of a coated PBS increases system cost and complexity and also requires additional alignment processes. Development of better coatings that provide higher extinction levels may be able to achieve high extinction ratios in coated PBSs, but methods for improving the extinction ratios of coated PBSs without increasing system cost and complexity are sought.